This invention relates to apparatus for generating a video signal representing a field of spatially varying color.
A video mixing system, or switcher, may be used to combine video signals from external video sources to provide an output signal representing a picture in which there is a transition between the image represented by one external signal and the image represented by the other external signal. Such a transition may be accomplished by use of a wipe. In a wipe, the switcher receives input video signals representing two images and changes from one input video signal to the other in response to a wipe signal, which defines a predetermined wipe pattern. FIG. 1 illustrates a simple split screen wipe, where the left side of the output picture is the left side of one input image and the right side of the output picture is the right side of the other input image. In this case, a wipe signal having a ramp waveform, which is a low voltage in areas corresponding to the left side of the picture and high voltage in areas corresponding to the right side of the picture, as shown by waveform A in FIG. 2, is generated. This wipe signal becomes the input to a clip and gain circuit (FIG. 3). The clip and gain circuit comprises a comparator 2, in which a clip level is subtracted from the voltage of the wipe signal, and the resulting difference signal (waveform B) is amplified by a multiplier 4 to provide an output signal (waveform C) which is level shifted at 6 and limited at 8 to provide a mix signal (waveform D). The mix signal is applied to the control input of a mixer 10 that receives the external video signals at its two video inputs. The clip level is under operator control and sweeps through a range of values as a manually-operated control, such as a lever arm, is swept through a region between the two In this way, the transition region between the two component images may be moved horizontally, e.g. from left to right of the field. It is possible to vary the width of the strip over which the transition between the two images takes place by adjusting the gain control signal applied to the multiplier. The wider the strip, the "softer" is the transition.
Conventional wipe generators are able to provide a wide variety of wipes in addition to the horizontal wipe described with reference to FIG. 1, such as rectangular wipes and annular wipes.
In a video effects apparatus, it is conventional for the input video signal to have an associated key signal. The key defines an area of the picture field. The value of the key lies in the range from zero to one. When the video signal is multiplied by the key, the video signal is driven to zero at locations for which the value of the key is zero.
A switcher generally includes several internal matte generators, each of which generates a full field video signal representing a single color. These matte generators may be used as general purpose video sources. One use of a matte signal is to accentuate a transition between images provided by external video sources. In FIG. 4, a wipe signal which represents a vertical line on the video field at a position that depends on a clip level, as in the case of FIGS. 1 and 2, is applied to a first clip and gain circuit 12. In the circuit 12, a first clip level is applied to the wipe signal to generate a mix control signal that is applied to a first mixer 13 to control mixing of a first external video signal (video A) with a matte signal. In a second clip and gain circuit 14, a second clip level is applied to the same wipe signal to generate a mix control signal that is applied to a second mixer 15 to control mixing of the output signal provided by the first mixer 13 with a second external video signal (video B). As shown in FIG. 5, in which field 16 represents video A, field 17 represents the matte and field 18 represents video B, the output picture represented by the signal provided by the second mixer 15 (field 19) is composed of portions of the images represented by the two external video signals, separated by a band of the matte color. The two clip levels may be created by adding offsets of equal magnitude and opposite sign to the clip level used in the case of the simple horizontal wipe, so that as the latter clip level is adjusted (and the offsets remain constant) the band sweeps across the field. If the gains of the two clip and gain circuits 12, 14 are different, the softness of the transition from one image to the matte color is different from the transition from the matte color to the other image.
A signal representing a matte of spatially varying color may be generated by mixing two or more flat matte signals under control of a wipe signal. However, this involves using a substantial part of the resources of a typical switcher and therefore limits the effects that can be performed by the switcher simultaneously with generation of the matte signal.
Moreover, if a signal representing a matte of spatially varying color is used in the circuit shown in FIG. 4, to accentuate a transition between two external images, the distribution of color over the matte portion of the output picture changes as the clip levels change, and this may divert attention from the external images.